The answer is the `passive-interface default` command configured under the EIGRP process for VRF_SALES. When this global command is issued, it automatically sets all interfaces under that EIGRP process to passive, which prevents them from sending or receiving EIGRP hello packets. Without hello packets, neighbor adjacency cannot form, which is exactly why the serial interface shows as passive in the `show ip eigrp vrf VRF_SALES interfaces` output and no neighbors appear. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how VRF-Lite interacts with EIGRP interface states, and it is a common trap because engineers often forget that `passive-interface default` overrides any individual interface configurations unless explicitly negated with `no passive-interface`. A quick memory tip: think of "default passive" as a blanket that smothers all EIGRP hellos—you must lift it off specific interfaces with `no passive-interface` to restore neighbor formation.
300-410 VRF-Lite Practice Question
This 300-410 practice question tests your understanding of vrf-lite. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A network engineer is troubleshooting a VRF-Lite setup where two routers are connected via a serial link. Each router has VRF_SALES configured. The engineer configures EIGRP in VRF_SALES. The 'show ip eigrp vrf VRF_SALES neighbors' shows no neighbors. The 'show ip eigrp vrf VRF_SALES interfaces' shows the serial interface is passive. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The 'passive-interface default' command is configured under the EIGRP process for VRF_SALES.
If an interface is marked as passive in EIGRP, it will not send or receive hello packets, preventing neighbor formation. This is a common misconfiguration.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
The 'passive-interface default' command is configured under the EIGRP process for VRF_SALES.
Why this is correct
This command makes all interfaces passive by default, and if the serial interface is not explicitly set to no passive, it will remain passive.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The 'network' command for the serial interface's subnet is missing.
Why it's wrong here
Missing network command would prevent EIGRP from running on the interface, but the interface would not show as passive.
✗
The 'autonomous-system' number is different on the two routers.
Why it's wrong here
Different AS numbers would prevent neighbor formation, but the interface would still be active, not passive.
✗
The 'metric weights' command is misconfigured.
Why it's wrong here
Metric weights affect route calculation, not neighbor discovery.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Trap categories for this question
Command / output trap
Missing network command would prevent EIGRP from running on the interface, but the interface would not show as passive.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
VRF-Lite — This question tests VRF-Lite — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The 'passive-interface default' command is configured under the EIGRP process for VRF_SALES. — If an interface is marked as passive in EIGRP, it will not send or receive hello packets, preventing neighbor formation. This is a common misconfiguration.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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